I’m running a dual boot (Win7 / Win2008) on my laptop and after I installed Windows 7 RTM this week-end, the boot configuration was messed up and I had to execute a repair to get Windows Server 2008 back in my boot menu. But apparently these boot configuration changes caused also Hyper-V to stop working : “virtual machine could not be started because the hypervisor is not running”.

To add the hypervisor auto launch back into the BCD store you’ll need to run the following command in administrator mode
bcdedit /set hypervisorlaunchtype auto